1. Field of the Invention
This invention relates to an electrophotographic photosensitive member by use of an organic photoconductive material, above all a laminated type electrophotographic photosensitive member having a photosensitive layer with a laminated structure having a charge generation layer and a charge transport layer provided by coating thereon.
2. Related Background Art
The photosensitive layer surface of an electrophotographic photosensitive member receives electrical, thermal and further mechanical external force by various processes in an electrophotographic copying machine such as corona charging, toner development, transfer onto paper, cleaning treatment, etc. Accordingly, the following problems have ensued by practicing repeatedly an electrophotographic process. First, through the mutual interaction between the paper powder generated from the transfer paper under highly humid conditions, ozone generated by corona discharging and further nitrogen oxides, etc., formed by ozone, low resistance materials are adhered on the surface of the electrophotographic photosensitive member. Whereupon there occurs a phenomenon that the image formed becomes indistinct as it blurs (hereinafter called "image flow"). Next, there are problems such as elevation of the residual potential accompanied by carrier trapping formed within the charge transport layer and at the interface between the charge transport layer and the charge generation layer. Also, a phenomenon occurs that the charge transporting material in the vicinity of the photosensitive layer surface is deteriorated by ozone to become reversibly low in resistance whereby the image becomes blurred (hereinafter called "unfocused image"). Further, friction damage and abrasion by mechanical external force also occur. These will all bring about lowering in image quality, whereby no stable sharp image can be constantly obtained.
The problems as mentioned above as influenced greatly by the characteristics of the binder resin contained in the charge transport layer in an electrophotographic photosensitive member having a charge transport layer laminated on a charge generation layer.
On the other hand, each layer of such an electrophotographic photosensitive member is generally formed by coating, and the binder resin has great influence on its coatability, and therefore production stability.
Accordingly, selection of the binder resin for a charge transport layer is very important, and at the same time, it is extremely difficult to find a material which can satisfy all of the demands as mentioned above. As the material to be used as the binder resin, there have been heretofore proposed methacrylic resins, acrylic resins, polystyrene, polyester, polycarbonate, polyarylate, polysulfone, etc., either as single product, or copolymers or blends. Among them, polycarbonate resins have overall excellent characteristics and have been practically applied. When a polycarbonate resin is used as the binder resin, there is little deterioration of electrophotographic characteristic such as residual potential elevation accompanied with repeated copying, whereby stable latent images can be obtained. Also, due to excellent abrasion resistance, durability is also excellent against various kinds of mechanical external forces. However, the surface layer containing a polycarbonate has a drawback in that the image flow phenomenon is very liable to occur. This may be considered because the low resistance materials adhered on the photosensitive member surface which causes image flow can be removed with difficulty due to the abrasion resistance of polycarbonate. Thus, this is nothing but the adverse effect of the excellent mechanical durability of polycarbonate. For solving this problem, there must be employed a means to provide a dehumidifying heater for the photosensitive member, or a means to clean the surface by roller cleaning. Accordingly, there ensues the problems such as increased costs, increase of power consumption, temperature elevation, image irregularity due to cleaning roller, etc., accompanied with such means. Particularly, when it is aimed to make the electrophotographic copying machine smaller and widely spread, employment of image flow preventing means as described above is further difficult also in aspect of space.
On the other hand, another drawback of a polycarbonate containing photosensitive layer is the point that unfocused image is liable to occur. While it is not clear how the polycarbonate which is the binder resin participates in ozone deterioration of the charge transport material, it may be considered to have an effect on arrangement and orientation of the charge transport material in the charge transport layer, which makes the vicinity of the surface conspicuously lower in resistance when subjected to ozone deterioration. For solving this problem, it is necessary to maintain constantly the ozone concentration at a low level by controlling the evacuation state around the photosensitive member. Accordingly, new factors for enlargement of copying device such as enlargement of evacuation fan and motor, arrangement of evacuation duct as well as increased costs are further provided.
As described above, when using a photosensitive layer containing a polycarbonate resin, there has been the drawback that the above problems occurred during attempts to make an electrophotographic copying machine on a small scale, at lower costs, thereby requiring suitable expensive countermeasures.